Gear systems that match natural output characteristics of a source of mechanical power with requirements of a load are known. Gearboxes that function to trade speed for torque and vice versa, may be used in a range of applications, for example to permit a high speed, low torque electric motor to move a heavy load slowly or to match the characteristics of a wind turbine.
Planetary gear systems with planet elements that may be carried on a planetary carrier and rotate individually as the planet elements rotate about a central axis and roll against concentric elements have been used in many applications. Mechanical input or output in a planetary gear system may be provided by spinning a concentric gear or a planet carrier. In some planetary gear systems planet elements may be stepped or compound, with each planet element being formed with two different gear surfaces. Each gear surface has a different diameter and is constrained to rotate at the same speed. The different gear surfaces are typically associated with and roll against respective concentric elements. When the diameter of the planet elements is smaller than that of the concentric elements, the planet elements rotate at a higher speed than any of the other elements in the system. Planetary gearing with such stepped or compound planet elements may develop an extremely high gear ratio with only a few gear stages. Planetary gear systems may be constructed with gear system elements that have only toothed elements, known as spur gearing, gear system elements that have smooth surfaces, known as traction gearing, with a combination of toothed and smooth surfaces, and with other types of rotary elements.
In recent years, much effort has been directed toward moving vehicles efficiently on ground surfaces without using fossil fuel-powered or internal combustion engines. Providing one or more vehicle drive wheels that are powered by other sources, typically electric drive motors, can be an effective way to move a vehicle. In U.S. Pat. No. 7,445,178, for example, McCoskey et al. describes electric motors mounted to drive aircraft nose wheels, each of which has a planetary gear system with planetary and sun gears rotated through a ring gear by a rotor of the wheel motor. A harmonic drive designed to be driven by an electric motor and meet the torque and power requirements required in a compact landing gear wheel-mounted drive system capable of driving an aircraft during ground travel is described in commonly owned U.S. Pat. No. 9,233,752 to Walitzki et al. This system is described to produce a higher gear ratio within a smaller volume than is possible with planetary gears. In another illustrative known drive wheel arrangement, the components of an electric motor are fitted within the dimensions of the wheel and are driven by gearing to drive the wheel.
Gear systems used to drive motors are required to match the natural output characteristics (speed and torque) of the motors to load requirements. Optimally, electric motors naturally operate at high speed and low torque at a specific power level. Many loads, however, require high torque at low speed. Changing the gear ratio of a gear system to a higher ratio of input speed to output speed in this situation has been suggested. While a higher gear ratio may be produced by adding gear stages or by the use of a harmonic drive, these approaches lead to a reduction in gear system efficiency. It is desirable to maximize the efficiency of a gear system.
In a conventional gearbox, respective gear stages are arranged on parallel, horizontally offset axes, with one gear per axis. The axes are horizontally offset from each other by a selected distance so that the gear elements at a designated end of each axis make contact with and drive, or are driven by, each other. The sizes of the gear elements are chosen to achieve a desired ratio. For example, if an input gear element with a diameter of one inch is positioned on a parallel axis to drive an output gear element with a diameter of six inches, an advantage ratio of 6:1 will be achieved. It is, however, difficult to achieve a higher gear ratio within the space occupied by this type of gear arrangement. If a higher gear ratio can be achieved without sacrificing efficiency, the applications of gear systems can be expanded.
The art has not suggested a gear system or a gearbox with an arrangement of gear elements that effectively multiplies torque and increases the gear ratio of the system while maximizing efficiency. A need exists for such a gear system with an arrangement of gear elements that permits a motive input to drive planet gear elements in the system to increase the gear reduction ratio while maximizing efficiency of the gear system. A need further exists for an efficient gear system with an arrangement of planet gear elements driven by a motive power source that is capable of driving a vehicle wheel and moving the vehicle at desired travel speeds on a ground surface.